1. Field of the Invention
The present invention relates to organic photoconductive layers and specifically the protection of those layers and the extension of their useful life in imaging processes.
2. Background of the Art
Multicolor toner images produced by successive toner transfer from a photoconductor to a single receptor are well known in the art both for powder toners with constituents intended to improve resolution on transfer and for use with magnetic brush development (U.S. Pat. No. 3,833,293). U.S. Pat. No. 3,612,677 discloses a machine designed to provide good registration when using successive color image transfer, and U.S. Pat. No. 3,804,619 discloses special powder toners to overcome difficulties toners have in 3 color successive transfer.
The production of multi-colored images by overlaying toned images on a photoconductor surface is also known. Thus U.S. Pat. No. 3,337,340 discloses liquid developers designed to minimize the "bleeding away of charge on the photoconductor surface" which occurs when recharging of an already toned surface is attempted. U.S. Pat. No. 4,155,862 and U.S. Pat. No. 4,157,219 disclose liquid toner formulations and apparatus for producing multicolor composite toned images on a photoconductor surface. U.S. Pat. No. 4,275,136 emphasizes the difficulties in ensuring that overlaid toner layers on a photoconductor adhere to one another. The addition of zinc or aluminum hydroxides coated on the colorant particles is used to solve the problem. No transfer of composite images is disclosed in these references.
Many methods are used to aid the efficient transfer of toner from a photoconductor surface after toner development to a receptor sheet. U.S. Pat. No. 3,157,546 discloses overcoating a developed toner image while it is still on the photoconductor. A liquid layer having a concentration of about 5% of a film-forming material in a solvent is used at between 10 and 50 microns wet thickness. After drying, transfer is carried out to a receptor surface which has a mildly adhesive surface. Defensive Publication T879,009 discloses a liquid toner image first developed on a photoconductor and then transferred to a receptor sheet whose surface is coated with a polymer layer easily softenable by residual solvent in the developed image which thus adheres the image to the receptor surface. U.S. Pat. No. 4,066,802 discloses the transfer of a multitoned image from a photoconductor, first to an adhesive carrier sheet, and then to a receptor. The second stage involves the application of heat and pressure with a "polymeric or plasticizing sheet" between the image on the carrier sheet and the receptor surface. U.S. Pat. No. 4,064,285 also uses an intermediate carrier sheet which has a double coating on it comprising a silicone release layer underneath and a top layer which transfers to the final receptor with the multicolor image and fixes it under the influence of heat and pressure. U.S. Pat. No. 4,337,303 discloses methods of transferring a thick (high optical density) toned image from a photoconductor to a receptor. High resolution levels of the transferred images are claimed (200 1/mm). It is required to dry the liquid toned image and encapsulate the image in a layer coated on the receptor. Curing of the encapsulating layer is required with some formulations. The materials of this layer are chosen to have explicit physical properties which provide not only complete transfer of the thick toner image but also ensure encapsulation of it.
U.S. Pat. No. 4,477,548 teaches the use of a protective coating over toner images. The coating is placed on the final image and is not involved in any image transfer step. The coating may be a multifunctional acrylate, for example.
Transfer of certain types of composite multitoned images is disclosed in the art. U.S. Pat. No. 3,140,175 deposits microbeads containing a dye and a photoconductor on one electrode, exposes them through a colored original and then applies field between a first and second electrode causing separation of charged and uncharged beads and transfer of the colored image to a receptor surface at the second electrode. U.S. Pat. No. 3,376,133 discloses laying down different colored toners sequentially on a photoconductor which is charged only once. The toners have the same charge as that on the photoconductor and replace the charge conducted away in image areas. However, it is disclosed that subsequent toners will not deposit over earlier ones. The final image of several toners is transferred to a receptor and fixed. U.S. Pat. No. 3,862,848 discloses normal sequential color separation toned images transferred to an intermediate receptor (which can be a roller) by "contact and directional electrostatic field" to give a composite multitoned image. This composite image is then transferred to a final receptor sheet by contact and a directional electrostatic field.
U.S. Pat. No. 4,600,669 describes an electrophotographic proofing element and process in which successive liquid toned color images are formed on a temporary photoconductive support. The composite image is then transferred to a receptor layer. The photoconductive layer has a releaseable dielectric support coated thereon which may comprise a polymeric overcoat on the photoconductive layer which is transferred with the composite image.
U.S. Pat. No. 4,515,882 describes an electrophotographic imaging system using a member comprising at least one photoconductive layer and an overcoating layer comprising a film forming continuous phase of charge transport molecules and charge injections enabling particles.
Protective overcoating layers have been proposed for the purpose of enhancing the durability of electrophotographic photoreceptors. For example, the imaging surfaces of many photoconductive elements are sensitive to wear, humidity, ambient fumes, corona induced changes, scratches and deposits which adversely affect electrophotographic performance. In addition, auxillary layers designed to control specific properties such as light absorption or dark discharge rate have also been described. However, many of the overcoating layers adversely affect the electrophotographic responses of a photoreceptor construction. For example, when an electrically insulating top-coat is used, there is a tendency for a residual potential to remain on the photoconductive member after exposure where the intensity of this residual voltage increases with the thickness of the insulating coating. In many cases, this residual potential shows a tendency to increase as the photoreceptor is cycled, which can make the development process difficult to control. To minimize such problems, the insulating layer must be made extremely thin; but this can limit their efficiency since they are then easily damaged and subject to rapid wear. Attempts have been made to overcome these difficulties by the use of overcoats having higher levels of electrical conductivity, for example, by including quaternary ammonium salts in the topcoat. However, the conductivity of such layers is typically highly dependent on ambient moisture. Under very dry conditions, the conductivity of these layers may diminish to the extent that they show the same limitations as insulating materials. At high humidities, lateral charge migration can lead to loss of image resolution.
A further variety of overcoats for electrophotographic photoconductors involves the use of a layer having a low surface energy; the purpose of such a layer being to increase the efficiency of toner transfer from the surface of the photoreceptor. silicon and fluorocarbon polymers have been previously described as effective for this application. However, when such materials are solution coated, the solvent used can leach active materials from the OPC film resulting in adverse effects on both photoresponse and on the release properties of the topcoat. Moreover, such release films frequently require thermal "cure" at temperatures exceeding the glass transition temperature of the underlying OPC matrix during which materials from the photoconductor can migrate into the overcoated film.
U.S. Pat. No. 4,565,760 describes a photoresponsive imaging member comprising a photoconductor layer and, as a release protective coating over at least one surface, a dispersion of colloidal silica and a hydroxylated silsesquixone in alcohol medium.
U.S. Pat. No. 4,600,673 describes the use of silicone release coatings on photoconductive surface to increase the efficiency of toner transfer in electrophotographic imaging processes.
U.S. Pat. No. 4,721,663 describes an improved enhancement layer used in electrophotographic devices between a top protective layer and the photoconductor layer.
U.S. Pat. No. 4,752,549 describes an electrophotographic receptor having a protective layer consisting of a thermosetting silicone resin and a polyvinyl acetate resin. The combination provides improved densability.
U.S. Pat. No. 4,510,223 describes a multicolor electrophotographic imaging process. A general description of transfer of the toned image to an adhesive receptor is disclosed (column 15, lines 21-40).
U.S. Pat. Nos. 4,323,591; 4,306,954; 4,262,072; and 4,249,011 relate to polyacrylate materials having heterocyclic nuclei and processes for their cure into hard, solvent-resistant and abrasion-resistant films. These monomers are curable out of solvent-free compositions and can be cured by irradiation in air.